Per google, this appears to happen regularly. A barn goes up iin flame with about 100 K hens. Hillandale has about 20 million hens.
Omg. Did not know this was not unusual. Our son just started as a management trainee at an egg farm with about 1 million chickens. He better get up to speed on fire safety.
most likely electrical short or spontaneous combustion. A pile of chicken poop can get very hot very fast.
https://www.chicagotribune.com/nation-world/ct-manure-pile-fires-20170114-story.html